Transcript Oceanography Chapter 12

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Sediment Study Tools and Techniques
Chapter 12 Pages 12-3 to 12-6
The Study of Sediments
 Techniques and tools to study ocean
sediments include:
 Clamshell sampler – collects a large
sample of the top sediment.
 Piston corer – used to collect different
sediment layers as deep as 25 meters
(82 feet) into the ocean bottom.
 Specialized vessels with drilling
equipment – used to produce
sediment core samples of 500 meters
(1,640 feet) long.
 Seismic tools – transmit sounds that
travel through water and into sediment.
Different sediment layers create distinct
echoes. Used when looking
for oil or natural gas.
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Stratigraphy and Paleoceanography
 Scientists estimate there are no sediments on the ocean crust dated older than
200 million years.
 Study of sediment layers is called stratigraphy.
 A new science called paleoceanography is the study of prehistoric oceans.
Chapter 12 Pages 12-6 to 12-9
The Study of Sediments
 Scientists use deep-sea stratigraphy to look for clues, such as rock composition,
microfossils, deposition patterns and other physical properties.
 Based on these they can estimate the age of the sediment layers and draw
conclusions about the past.
 Ocean scientists use stratigraphy to understand changes in the ocean and
atmosphere; previous circulation patterns, former sea levels, and trends in
biological productivity.
 From sediment they have been able to estimate prehistoric ocean temperatures and
climatic conditions with accurate precision.
 Ongoing research of the Earth’s ancient climate currently emphasizes deepocean sediments called siliceous oozes.
 Oceanographers have played a key role in helping develop current theories that
propose that Mars once had oceans.
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Sediment Origins
 Sediments may be classified by origin based on four categories.
 1. Lithogenous sediments – Come primarily from land erosion carrying particles into the
sea or from volcanic eruptions. These are the majority of sediments.
 Quartz and clay are the two most common materials along with feldspar. Volcanic islands
contribute sediments of basalt and other volcanic materials.
 2. Biogenous sediments – Originate from organisms and cover a large area
of the seafloor.
 Silica and calcium carbonate are the materials that come from shells and hard skeletons of
planktonic organisms.
 3. Hydrogenous sediments – Result from chemical reactions within seawater accounting
Chapter 12 Pages 12-10 to 12-13
Types of Sediment
for less than 1% of the seafloor sediments.
 Minerals come out of solution and form particles that settle on the bottom.
 Produces ferromanganese and phosphorite nodules.
 Sources of dissolved minerals vary – submerged rock, new crust formation, hydrothermal vent
water, river runoff.
 4. Cosmogenous sediments – Come from outer space consisting of cosmic dust and
occasional impacts from asteroids and comets.
 They settle through the air as the others do through water. About 15,000-30,000 metric tons of
space dust settle on the Earth each year; least abundant of the sediments.
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Sediment Sizes
Chapter 12 Pages 12-13 to 12-15
Types of Sediment
 Sediments are classified on grain size – the
diameter of the particle.
 Grain size and current velocity affect the
deposition and erosion of sediment.
 Smallest and largest particles behave similarly with
respect to transportation and erosion.
 Sand in the middle of the graph takes the least amount
of energy to erode.
 Larger particles require more energy to erode because
they’re heavy. It takes a stronger current to lift them off
the bottom.
 Particles smaller than sand also take more energy to
erode. Smaller particles (especially clay) tend to be
cohesive.
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Sedimentation Processes on the Continental Shelf
 Tides, waves, and currents strongly
affect continental-shelf sedimentation.
Recent and Relict Sediments
Chapter 12 Pages 12-16 & 12-17
Continental-Shelf Sediments
 Shoreline turbulence: waves are one of the
most notable influences because it keeps
particles from settling. Surf and waves carry
small particles out to sea. Their affect
diminishes further from shore traveled.
 Recent sediments have accumulated since
the sea level stabilized.
 Relict sediments accumulated and were
left stranded when the sea level was lower.
 Overall, sedimentation on the shelf is more
rapid than in the deep ocean.
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Continental-Shelf Sedimentation Rates
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The sedimentation rate on the continental shelf varies with region.
Sedimentation on the shelf is more rapid than
in the deep ocean.
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Continental-shelf sedimentation processes also
affect the adjoining deep ocean.
Chapter 12 Pages 12-17 to 12-19
Continental-Shelf Sediments
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At the mouths of large rivers, sedimentation can
occur at a rate of one meter per thousand years,
but there’s a lot of variation.
Accumulating sediment on the continental shelf
avalanches down the continental slopes. These are
called turbidity currents and can carry sediment
deposits all the way to the abyssal plain. These
deposits are called turbidites.
Turbidites consist of layers of lithogenous sand embedded with the more typical, fine
deep-sea sediments.
The continental shelves undergo processes that produce biogenous sediments, which also
affect the sedimentation rate.
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Continental-shelf sediments tend to have a mix of both biogenous and lithogenous materials.
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Sedimentation Processes on the Deep-Ocean Bottom
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Chapter 12 Pages 12-20 & 12-21
Deep-Ocean Sediments
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Like the processes that affect the continental shelf, sedimentation processes in the deep
ocean vary regionally.
Deep-ocean sediments tend to be high in biogenous material.
Lithogenous sediments, except for clays,
are generally confined near shore.
Biogenetic sediments – primarily the
remnants of plankton – dominate the
sediments off shore waters.
Because of its very small grain size,
clay can remain suspended in the water
for great distances and be carried by wind,
allowing it to deposit in the deep sea.
The variation in deep-water sedimentation causes
tremendous variations in sediment accumulation.
The thickness of sediments in the deep ocean also varies with topography.
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Sediments are thickest on the abyssal plains and thinnest or absent on the mid-ocean ridges
and seamounts.
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The Carbonate Compensation Depth
 The carbonate compensation depth is a
point at which calcium carbonate dissolves just
as fast as it accumulates from above.
Chapter 12 Pages 12-22 & 12-23
Deep-Ocean Sediments
 Above this depth calcareous ooze dominates.
Siliceous ooze dominates sediments below this
depth due to the slow deep-sea dissolution of
siliceous remains and high diatom productivity.
 The carbonate compensation depth varies with
region due to temperature and water density.
 In the Atlantic and Pacific, it is around 4,500 meters (14, 750 feet).
 In colder regions, the carbonate compensation depth is much shallower so siliceous oozes
dominate biogenous sediments in polar regions.
 The slow dissolution of siliceous remains and high diatom productivity allow
siliceous oozes to accumulate throughout the seafloor.
 Siliceous ooze are the dominant biogenous sediments below the calcium carbonate
compensation depth.
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Fecal Pellets
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Scientists find that bottom composition is usually similar to the particle composition of the water above it.
This is due to fecal pellets.
Large planktonic organisms, like copepods, consume the calcareous or silicone organisms that also
dominate the bottom ooze. These large organisms eliminate their waste as dense fecal pellets of
multiple skeletal and shell remains compressed together. These dense pellets sink quickly and the
decomposition process begins.
Mineral Nodules
Chapter 12 Pages 12-23 & 12-24
Deep-Ocean Sediments
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Ferromanganese nodules consist of iron and manganese
found over as much as 50% of the deep Pacific floor.
Phosphorite nodules consist of phosphorite and other trace
minerals found on the shallow banks and continental shelves
off California, Argentina and Japan.
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Both forms of nodules are thought to be hydrogenous
sediments produced by one of the slowest chemical reactions in nature.
Nodules grow at a rate of about 1 to 200 millimeters (.039 to 7.9 inches) per million years.
Scientists believe that biological processes – possibly involving bacteria – cause the
chemical precipitation.
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Petroleum and Natural Gas
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What is the economic importance of ocean sediments and their study?
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Ferromanganese and phosphorite nodules have potential economic value.
Other Sediments With Economic Importance
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Chapter 12 Pages 12-25 to 12-27
Sediments as Economic Resources
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Oil and natural gas found under the ocean contribute $125 billion in annual revenues. More
than a third of the world’s crude petroleum and a quarter of its natural gas come from
sedimentary deposits on the continental shelf.
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Metal sulfide deposits found at deep-sea hydrothermal vents are rich and vast enough
(especially in the Red Sea) that mining them could be economically feasible.
Evaporites form at the surface and comprise the salts left behind when seawater
evaporates. They are a source of calcium carbonate, calcium sulfate, gypsum and
sodium chloride.
Sand and gravel are an important resource for the
construction industry accounting for $500 million yearly.
Another sediment-based resource is diatomaceous earth.
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